In reactors that generate high molecular weight hydrocarbons (wax), some of the reaction products must be removed from the reactor in liquid form, preferably without losing catalyst, which will be in suspension in a finely divided form.
It is well known that there is a high risk of generating fines in a FT slurry reactor due to attrition/erosion of catalyst, and this can create the danger of catalyst loss and blocking of wax filters by fine catalyst particles.
Several methods have been described in the literature for effecting the separation of the catalyst and/or wax from the reactor slurry, such as internal filtration, external filtration, hydrocyclones, magnetic methods and settling/sedimentation. As the concentration of catalyst fines builds up in the reactor slurry, the flux through the filter will decrease and at a certain point it will be too low to maintain the filtrate capacity needed for a given wax production rate.
There are several known ways of reducing the effect of fines on the filtration, such as using a larger filtration area, more frequent cleaning (for example back-flushing) of filters, higher differential pressure for filtration etc. However all of these methods are either space limited or energy consuming and may shorten the life time of the filters.
The relationship between fines concentration in the slurry and the filtration flux is highly non-linear. This is shown as a very large decrease in flux as a result of increasing the fines concentration at very low or virtually zero fines concentrations, while further increases in fines concentration has a smaller effect. The fines generation rate in the reactor is not particularly sensitive to the wax production rate, but is more sensitive to the fluid dynamics of the reactor, which are for example related to the feed gas rates and of course the mechanical properties of the catalyst. Operating the reactor at conditions that give lower wax production is not a permanent solution; it simply extends the operating time somewhat and gives lower overall production. The concentration of fines in the slurry will increase at a higher rate if the removal of wax is decreased as the only outlet of fines is through the filtered wax.
It is therefore important to have a system for controlling the fines removal rate out of the reactor, both during normal operation and during upset conditions/turn down.